Evaluate String as a condition Java

It's not going to be trivial: you need a parser for the expression language used in your database. If it's some standard, well-specified language, then you might be able to find one on the Internet, but if it's an in-house thing, then you may need to write your own (perhaps using a parser generator like ANTLR.)

The javax.script package contains some tools for integrating external scripting engines like a Javascript interpreter. An alternative idea would be to bring in a scripting engine and feed the expressions to that.

You are basically evaluating a scripted expression. Depending what is allowed in that expression, you can get away with something very simple (regular expression identifying the groups) or very complex (embed a javascript engine?).

I'm assuming you're looking at the simple case, where the expression is: [boundary0] [operator] "value" [operator] [boundary1]

Where one, but not both of the [boundary] [operator] groups might be omitted. (And if both are presents, operator should be the same)

I would use a regular expression for that with capturing groups.

Something like: (?:(\d+)\s*([<>])\s*)?value(?:\s*([<>])\s*(\d+))?

And then: boundary1 = group(1); operator1 = group(2); operator2 = group(3); boundary2 = group(4)

Here is an example using the standard (Java 1.6+) scripting library:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class Test {

    public static void main(String[] args) throws Exception {
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("JavaScript");

        engine.eval("value = 10");
        Boolean greaterThan5 = (Boolean) engine.eval("value > 5");
        Boolean lessThan5 = (Boolean) engine.eval("value < 5");

        System.out.println("10 > 5? " + greaterThan5); // true
        System.out.println("10 < 5? " + lessThan5); // false
    }
}